Investigating the Solvent Barrier to Interfacial Electron Transfer as a Function of the Ionic Strength

Interfacial electron transfer reactions have longstanding importance in the electronics industry and those reactions initiated with light are of particular importance in solar energy conversion, guiding best practices for dye-sensitized solar cell and photoelectrochemical cell design. Herein, the kinetics of charge recombination between a mesoporous film of tin-doped In2O3 (ITO) nanoparticles and one-electron oxidized [RuII(bpy)2(4,4’-(PO3H2)2-bpy)] 2+ (RuP) where bpy = bipyridine have been quantified under the effects of increasing ionic strength in acetonitrile solutions of LiClO4 and tetrabutylammonium hexafluorophosphate (TBAPF6). The ITO surface was coated with a 5 Å layer of ZrO2 by atomic layer deposition before sensitization with RuP so that charge transfer from reduced ITO to oxidized RuP could be time-resolved on a nanosecond to microsecond timescale. It is shown, however, that no measurable difference in the reorganization energy occurs when the electrolyte concentration is changed from 50 mM to 1 M in acetonitrile solutions. There is a small, but significant difference of ~ 0.05 eV in the reorganization energy for charge recombination when switching between 0.1 M LiClO4 and 0.1 M TBAPF6 indicating that the ions contribute to the outer-sphere reorganization energy.Motivation for this thesis research arises from the dependence of the Gouy-Chapman-Stern model of the electric double layer on the ionic strength and literature precedent. Both indicate that at the electrode-solution interface, solvent and electrolyte dipoles are rigidly oriented in the direction of the electric field. This has provided outer sphere reorganization energies which are much less than would be otherwise predicted by dielectric continuum theory. Under the effects of increasing ionic strength, the ability of dissolved ions to screen the electrode’s field is predicted to increase, thereby raising the outer-sphere reorganization energy to electron transfer. However, this effect was not observed under the conditions studied. A definitive explanation for this behavior is lacking yet suggests that electrolyte ions are unable to screen the electric field experienced by the redox active probes.